Your SB-600 is already compatible with Nikon's CLS wireless (infrared) flash control system. It's a pretty good system to build around; you get TTL (ie: automatic exposure), multiple flash capability, and remote control of your flash level. I own one of these flashes myself, and it's definitely a nice option to have.

Unfortunately, your current D3000 body doesn't support CLS. (My D90 does, as does any of the bodies you mention in your upgrade plans). You can buy an add-on adapter (Nikon SU-800)(http://www.nikonusa.com/Nikon-Products/Product/Flashes/4794/SU-800-Wireless-Speedlight-Commander.html) that will act as a "Commander" for your CLS-enabled flash.

I personally wouldn't spend money on it, depending on how soon you plan to upgrade. CLS is nice, but there are downsides:

Nikon-only. You generally can't use 3rd-party flashes with it.

Preflashes. The flash flickers a couple of times before the actual exposure happens. This introduces a delay, can potentially trigger blinks in people, and can interfere with optical triggers (which other posters have discussed).

Need line-of-sight, which won't work in all lighting situations.

If these aren't a big deal to you, then you might want to build around a CLS system. If that's true, and you want to go off-camera right away, and you don't mind spending some extra money, then the SU-800 is a great way to start. Note that it will be compatible with your new body. It'll be unnecessary (since those cameras use the built-in pop-up flash as a CLS commander), but still useful (you get extra control options on the standalone commander unit).

Actually, and I must add that I would like to find the opportunity to test this, the SU-800, coupled with a radio trigger like a pocket wiazard or similar system, should allow you to use the CLS system with any other camera as well, provided there is a radio trigger fitting the camera you are using... Also, I find that the added flash from the pop-up flash, even when set to -- in commander mode, can affect some exposures negatively, so, given the budget, I may eventually get an SU-800 to use with my D90 myself...
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RolandSep 5 '11 at 8:17

Thanks - the CTR-301p is more in the budget range I was thinking about. I don't want to spend £200+ when I won't need it after upgrading the body. Thanks to the other posters too :-)
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codecowboyMar 27 '11 at 5:15

@codecowboy - note that the regular optical slaves and the CTR-301P's optical slave are not compatible with Canon flashes, due to the pre-flash triggering them. You ask about Nikon equipment so you should be fine. There are slaves compatible with Canons. The I own the CTR-301P and used it successfully with the RF mode on my Canon equipment.
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ysapMar 27 '11 at 5:26

@codecowboy - I'm curious, though, why you are interested in such when you have a SB600? Doesn't it give you the wireless control you need built-in?
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ysapMar 27 '11 at 5:27